This invention relates to a photographic objective provided with a mechanism for preventing changes in angular field of view during focusing, and more particularly to such mechanism for a zoom lens.
As one form of provision for focusing, it is known to axially move the front part of a lens system which is arranged on the object side. Particularly in the case of the zoom lens, since the image shift is prevented from occurring with zooming by always giving an object point (an image point of the focusing lens) at a fixed position relative to the zooming lens groups such as variator independently of the object distance, the provision for focusing is generally made not in the entire lens system but at a front lens group (focusing lens). Also for long focal length objectives, it has been proposed to focus by moving of the front lens group for the purpose of avoiding an increase in size and weight of the lens mounting which would be otherwise caused when the entire system is moved to effect focusing.
In this front lens group dependent focusing method, for purposes of minimizing the diameter of the front lens member and of shortening the object distance, it is known to divide the front lens group into two parts which are moved relative to each other during focusing. This type focusing mechanism is disclosed in U.S. Pat. No. 3,632,188 where negative and positive lens subgroups are moved in the same direction by different amounts during focusing, Japanese patent application Laid-Open No. Sho 49-7752 where the focusing lens group is comprised either of two lens subgroups having negative and positive powers, or of a negative, positive and a further negative subgroup, and the front two members are made movable in opposite directions to each other. The technique is also disclosed in U.S. Pat. No. 4,124,274 where the focusing lens group is comprised of a negative, positive and a further positive subgroup, and the front two members are made movable in opposite directions to each other.
In the focusing of lens systems and particularly of zoom lenses, a problem that is particularly desired at present to be solved, is the minimization of variation of angular field of view with focusing.
Conventionally, various proposals have been made to suppress the variation in the angular field of view which occurs during focusing. In Great Britain Pat. No. 975,160 and Japanese patent application Publication No. Sho 42-12103, the front lens group of the zoom lens is constructed with two members of negative and positive powers respectively of which only the frontmost member of negative power is axially movable to effect focusing while achieving minimization in the variation of angular field of view. In U.S. Pat. No. 3,682,534, the front lens group is constructed with two members both of which are of negative power, and the variation of angular field of view is minimized by axially moving the second member from the front while maintaining the first or frontmost member stationary during focusing. Further, in U.S. Pat. No. 4,099,845, of the negative, positive and further positive members constituting the focusing lens group only the second positive member is movable towards the image side. Furthermore, in Japanese patent application Publication No. Sho 53-6051, there is disclosed the fact that the variation in the angular field of view is made nil by moving the two members of the focusing lens group under a certain condition, but the theory for its focusing is conducted with regard only to the front part. Since the remaining lens system that follows the front part is neglected, it would be difficult to utilize this focusing technique in a practical lens system.